Methods for making antibodies are well known and have become routine for most antigens. However, some antigens, due to small size, conformation changes under different conditions, or low immunogenicity—for example, highly conserved protein or proteins which are heavily glycosylated, have not been easy to make highly specific antibodies to.
A number of methods have been developed to address this problem. For example, it is well known that haptens or small molecules such as peptides and drugs are not immunogenic unless conjugated to a protein. Such proteins are designated as carrier proteins and such conjugated haptens as immunogens. However, it has been discovered that conjugation can alter not only the charge but also the conformation of the hapten, thereby generating antibodies that recognize the free hapten to a lesser extent than the immunogen.
The high immunogenicity of most linkers has also been a major obstacle to generating monoclonal antibodies for haptens of small size, e.g. cotinine, for which both the yield of useful clones and the affinities of available monoclonal antibodies are low. Thus, it has proved very difficult to raise monoclonal antibodies to many drugs and to manufacture peptide vaccines that will induce neutralizing antibodies to infectious agents.
Many commercial assays require highly specific antibodies, particularly for use in chromatographic assays where the result is to be indicative of a quantitative value, not just qualitative. For example, diabetes is a severe, life-threatening, chronic disease resulting from an impairment of the body's ability to turn glucose into usable energy. Type II diabetes is the most common form of diabetes. Up to 95 percent of the 16 million Americans with diabetes have Type II. It is also known as adult-onset diabetes, as it usually develops in people over the age of 45. In addition to age, weight and lack of physical activity or exercise, heredity also plays a role in a person's risk of having the disease.
Heart disease, stroke, kidney disease, blindness, circulatory and nerve problems are linked to long-term, high levels of blood sugar (hyperglycemia). Co-morbid conditions often include hypertension, high cholesterol and triglycerides. Hemoglobin A1c (HbA1c) testing has great importance in the overall management of diabetes since HbA1c reflects the portion of glucose that attaches itself to hemoglobin. It has been shown to accurately and reliably reflect long term levels (2–3 months) of chronic hyperglycemia. Therefore, while daily glucose monitoring is required for immediate intervention, HbA1c levels are considered a more accurate indicator of an individual's long term blood glucose levels. In addition to other in-office and at-home tests, the American Diabetes Association (ADA) recommends HbA1c testing four times a year for insulin-treated patients and at least twice yearly for all other patients with diabetes, or as often as needed to help achieve good glycemic control.
Just recently HbA1c has been approved for screening for diabetes. It is estimated that at some time during their lives approximately 10% of adults will develop adult onset diabetes. Most of these individuals are diagnosed after 10–15 years of hyperglycemia when the condition results in sugar in the urine. Damage is being done during the undiagnosed period. HbA1c screening could identify such individuals much earlier. Research shows that the HbA1c test can provide information that in many cases can help health care providers and patients develop regimens that dramatically lower the risks for serious and life-threatening diabetes complications, including blindness, kidney disease and nerve damage. Each year, diabetes results in 54,000 leg and foot amputations. Diabetes is the leading cause of end-state renal disease (kidney failure). It is the fourth leading cause of death by disease in the United States.
A landmark study known as the diabetes Control and Complication Trial (DCCT) revealed a direct correlation between high blood sugar levels and the development of long-term complications in people with Type I or insulin-dependent diabetes mellitus; there is no reason to believe that the effects of better control of blood glucose levels would not also apply to patients with Type II diabetes. The DCCT also found that, through blood glucose and regular HbA1c testing, adjustments could be made in diet, exercise or insulin dosage to reduce diabetes-associated risks. These include reductions in eye disease by up 76 percent, kidney disease by 56 percent and nerve damage by 60 percent.
Due to the complexity of existing HbA1c tests, they are generally performed in clinical laboratories and at significant costs. Since physicians treating individuals with diabetes rely on this test for the management of the patient's disease, it is desirable for it to be performed quarterly. Additionally, patients' interest in knowing their HbA1c number has increased largely as a result of the DCCT study. Most Type I (insulin dependent diabetic) know their HbA1c number just like they know their blood pressure or cholesterol level.
Hemoglobin A1c (Hb A1c) is one form of hemoglobin. It is identical to Hemoglobin A0 (Hb A0) with the exception that the N-terminal valine on the â chain is linked to C-1 of fructose through the amino group. This glycation causes a change in charge, which resulted in its first identification as the A1c fraction on an ion exchange column procedure. The formation of valine-fructose residue is believed to result from the formation of a Schiff base between valine and glucose followed by an Amadori rearrangement. The process is irreversible and the ratio of Ha A1c constitutes 4–6% of the total Hb. In diabetes patients, the ratio increases two to three fold to 6–15%.
The first step to develop an immunoassay in a Point of Care (POC) format to determine this ratio, i.e. Hb A1c/total Hb, is to develop an antibody that can discriminate between the native conformations of HbAo and Hb A1c. Critical to providing a test for screening are low cost reagents. The current assays for HbA1c entail expensive and/or cumbersome physical methods such as ion exchange and column chromatography, or almost equally cumbersome and therefore non cost effective immunoassays. Thus while antibody based assays have traditionally offered an economical alternative to physical methods, as will be discussed below, currently available antibodies do not offer the traditional advantages of specificity, economy and ease of use. Thus there is a need for an antibody that would offer ease of use, economy and specificity. Such an antibody would enable both Point of Care testing and adaptation of a HbA1c assay to any of many automated immunoassay systems.
There are several problems to be addressed when making an antibody to HbA1c or other antigens like HbA1c. The hemoglobin molecule is a poor immunogen because the hemoglobin sequence is highly conserved and it is difficult to overcome tolerance of self. The peptide sequence of the HbA1c epitope, hereafter the HbA1c epitope, is the same in mouse and human and most mammals: sheep have a different sequence in this epitope region of the N terminal. It is difficult to overcome tolerance of self. Even though most animals do not form HbA1c, it is not possible under normal conditions to use Hb A1c directly as the immunogen to make an antibody that can discriminate HbAo from HbA1c since the difference between A1c and Ao is only the addition of one glucose molecule. Fructose has low immunogenicity and so the dominant immune response is postulated to be to more immunodominant areas of the epitope.
A glycated site is not a good epitope: the HbA1c epitope comprises less than 1% of the hemoglobin surface. Therefore one must immunize with a peptide. It is difficult to make an antibody to a peptide that has high affinity for the peptide sequence of the native protein. The antibodies currently commercialized fall into two categories, polyclonal and monoclonal.
Boehringer Mannheim (BM) markets a turbidometric assay kit which utilizes a sheep polyclonal antibody. A sheep polyclonal prepared to HbA1c whole molecule is described by Javid et. Al. (Brit. J. Haematology: 38:329–337 1978) and U.S. Pat. No. 5,646,255 to Klein, et al. The BM antisera was raised to the reported immunogen sequence “Fructose Val His Leu Thr . . . ” (SEQ ID NO:1) (Karl, et al. Klin. Lab 39:991–61993). It is probable that this antibody can be successfully raised in sheep because sheep do not have the same amino terminal sequence as other mammals and hence they are able to recognize as foreign and immunologically response to the common mammalian N terminal sequence, “Val His Leu Thr” (SEQ ID NO:1). The mouse in which monoclonals are raised has the same sequence as humans and most other mammals and this probably explains why, when the same immunogen is used to immunize mice, that the majority, if not the only, antibodies that are produced, react with the denatured form of hemoglobin (which is foreign) but not the native conformation (which is not foreign). It should also be pointed out that all monoclonal antibodies are screened for in an Elisa format because that is the only truly economical method for performing all the screening that must be done during the course of making a monoclonal. The Elisa plate is coated with the hemoglobin or protein: hemoglobin is not in its native conformation, i.e., it denatures, when coated on an Elisa plate. Thus even if the mouse did produce a few clones that had the potential to recognize the native conformation, the screening process works to select for clones that recognize the denatured configuration and thus against selection of a clone that would recognize non denatured or native HbA1c.
The BM antibody does not show high specificity: In the experimental Elisa system described herein, this sheep polyclonal antibody shows that about 10–20% cross reacts with HbA0 and reacts equally well with native (i.e. HbA1c that is not specifically subjected to denaturing conditions) and denatured HbA1c. The BM antibody is commercialized in a turbidometric assay. These are liquid based assays that are run on an autoanalyzer and require less than 10 minutes. In such a system one can often disregard low level cross reactivity because cross-reactive moieties having lower association constants exert less interference in shorter assays. However, it would be difficult to utilize such an antibody in anything but an autoanalyzer assay, because to compensate for such high cross reactivity, all assay conditions, the time, pH, temperature, sample dilution, etc., must be carefully controlled, as they are on an autoanalyzer. In a point of care assay, it is not possible to dilute the sample or to add large amounts of buffering materials or to carefully control time and temperature. Thus cross reactivity becomes a much larger problem under assay conditions that are not ideal.
A further disadvantage of the BM polyclonal antibody is that it is not cost effective. The kit provides the exceptionally large amount, 40 μg, of purified antibody for each assay. Polyclonal antibody that must be purified and provided at such high concentration is very expensive. Point of Care assays, which usually require much larger amounts of antibody than laboratory assays, generally have less than 1 μg/assay. The large amount of antibody required by the BM test is probably due to low affinity of the antibody. Low affinity antibody generally means that the assay lacks a high degree of specificity. Indeed, it is well documented that all the currently commercialized immunoassays for HbA1c are lacking in specificity; i.e. they do not discriminate as well as required between various modifications of the N terminal valine.
Thus, while polyclonal antibody raised in sheep has the advantage that it can recognize native HbA1c sequence, it has the disadvantages that it is very expensive and that it lacks specificity and thus is limited to formats that can compensate for these restrictions on its performance. More importantly, polyclonal antibodies to one discrete conformational epitope cannot provide the consistency that is required by today's clinical laboratory standards, i.e. by definition, polyclonal antibodies contain many clones of different affinity and each animal at each bleed provides a mix of clones that unique to that bleed. When there is only one epitope and that epitope is conformational, polyclonal antibodies generally provide unacceptable variations in reagents from lot to lot.
A monoclonal antibody to HbA1c would have the potential to overcome the cost and consistency problem of polyclonal antiserum but thus far all available and described monoclonals recognize altered HbA1c and require either a 10 minute denaturation process or protease treatment to render the sample suitable for testing with the antibody. The requirement for pretreatment of sample precludes the adaptation of these antibodies to Point of Care Tests; non-laboratory personnel in a non-laboratory environment cannot be expected to treat the sample. Pretreatment also greatly limits the usefulness of such antibodies in screening assays and pretreatment adds significant cost and complexity to the test.
Monoclonal antibodies specific for the glucosylated N-terminal peptide residue in HbA1c are described in U.S. Pat. No. 4,727,036 to Knowles. These antibodies are hereafter referred to as the Miles Antibody. The Miles antibody was produced using the glycated N-terminal fragment of alpha chain as the immunogen. The first 15 amino acids of alpha chain N-terminal are Val-His-Leu-Thr-Pro-Glu-Glu-Lys-Ser-Ala-Val-Thr-Ala-Leu-Trp. (SEQ ID NO:2) The Miles Antibody was produced to a peptide sequence that included the first 8 amino acids: BM also utilized only the first 7 or 8 amino acids. Miles' HbA1c specific antibody is a monoclonal antibody which reacts only with denatured HbA1c (the kit requires 4 minutes with a chaotropic agent and it interacts with HbA1c immobilized on microtiter plate. The Miles antibody discriminates with great specificity between HbA0 and HbA1c when these proteins are denatured. This antibody shows almost no cross reactivity with HbA0 in our experimental Elisa assay. However it does not function as a diagnostic reagent unless the reagents are denatured. The patented immunogen for the Miles monoclonal is fructose-Val-His-Leu-Thr-Pro-Glu-Glu-Lys-Tyr-Tyr-Cys. (SEQ ID NO:3) (Tyr-Tyr is commonly used to obtain spacing of immunogen from the protein carrier). This is essentially the same immunogen described in the literature for producing polyclonal antibody to HbA1c and used by BM to produce sheep polyclonal. This monoclonal antibody recognizes the amino terminus in denatured HbA1c. Thus the sample requires pretreatment to allow interaction with the antibody. This pretreatment step renders the test of limited economic value. Further the literature indicates that the antibody recognizes other modifications of the terminal valine containing peptide: this indicates that the antibody recognizes modification of the N terminal, not the specific modification.
Generally speaking, antibodies made to linear peptide analogs of epitopes are of low affinity and thus lack specificity i.e. they will show high cross reactivity. In this case that means that the antibody made to MbA1c would be expected to recognize both HbAo and HbA1c. The monoclonal antibody described in patent U.S. Pat. No. 4,647,654 to Knowles, marketed by Miles, does not distinguish between Ao and A1c unless they are denatured; in the denatured form the peptide appears to the antibody as similar to the immunogen.
It is therefore a first object of the present invention to provide a method for making antibodies to immunogens that have low immunogenicity.
It is another object of the present invention to provide methods and reagents to enable generation of high titer antibodies to preferred epitope conformations, especially those where the conformation is altered by conjugation to carrier or by denaturation.
It is a second object of the present invention to produce a monoclonal antibody that reacts with antigens such as native HbA1c, that is both more accurate and sensitive than the antibodies used in currently available tests, and yet at the same time is cheaper to produce and use.
It is a further object of the present invention to provide an antibody that is useful in a point of care test and thus does not require any treatment and must react with the native molecule.
It is still another object of the present invention to provide a method and reagents to quickly and inexpensively measure antigens such as Hb A1c and to determine the ratio of antigens such as Hb A1c and Hb Ao.